Boost logo

Boost :

From: Gennadiy Rozental (rogeeff_at_[hidden])
Date: 2008-01-15 21:44:17

"John Torjo" <john.groups_at_[hidden]> wrote in message
> Hi all,

> * What is your evaluation of the design?

In short: in places too complicated, in places plain wrong.

In details:

1. mutexed_singleton

This should be singleton<synchronized<T> >. Current design mixes two
orthogonal concepts: uniqueness of class instance and class methods
synchronization and this is obviously wrong.

2. thread_specific_singleton.

The same as above. This should be singleton<thread_specific<T> >. Current
design mixes two orthogonal concepts: uniqueness of class instance and
thread specific object storage.

3. public construction and boost::restricted

I don't like this solution and very much prefer private constructor with
friend declaration (especially if encapsulated into single macro). This will
be a never ending source of confusion (what is boost::restricted, can I have
restricted access, how do I create restricted instances etc) and doesn't
really protect - once can still create instances of my class with valid C++

4. DLL support

Explanation of DLL issues in docs is very unclear. No examples provided. No
tests showing that solution actually works. Accordingly I can't really
comment on it, but from what I got I really doubt it.

5. instance proxy.

Given complete absence of rationale of this design, I can't really devise
why is was chosen, but it seems wrong. We end up with pricy complication
with unclear purposes. What's wrong with just returning T& from instance
method? The same time we can get rid of all this member_dereference staff
and it's limitations.

6. lease.

Pretty much the same as above. synchronized<T> should have it's own lock
facility. Otherwise instance() should be straightforward access to the
instance and no need for any proxies/leases.

7. MT support

Setting aside mutexed and thread specific singletons covered above, regular
singleton handling of MT is wrong as well. In simple words my primary
complain: you can't use define based solution for switching MT/non MT. It
should be template parameter. As of now there is no singleton that can work
in presence of threads but without disregard to them.

8. No Meyer's singleton

What if do not want all this manager staff?

> * What is your evaluation of the implementation?
> * What is your evaluation of the documentation?

As usual these days, scarce to the point of being useless. It also
incorrect - it doesn't cover third template parameter in reference for
example. Singleton is very basic and quite well known design pattern The
documentation should explain in details all design choices, compare with
different solutions and explain advantages. There are number of existing
pitfalls with singleton pattern - all have to be covered, starting with
double locking idiom problem and they it's solved (if they are).

> * What is your evaluation of the potential usefulness of the library?

Shaky. We'll either keep using trivial Meyer's-like singleton or need
solution which actually make sense and solve real problems. In later case it
still need require big disclaimer somewhere on top: do not overuse.

> * Did you try to use the library? With what compiler?


> Did you have any problems?
> * How much effort did you put into your evaluation?
> A glance? A quick reading? In-depth study?


> * Are you knowledgeable about the problem domain?


> And finally, every review should answer this question:
> * Do you think the library should be accepted as a Boost library?



Boost list run by bdawes at, gregod at, cpdaniel at, john at